Method and apparatus for seismicelectric prospecting



p 1936. w BLAU ET AL 2,054,067

METHOD AND APPARATUS FOR SEISMIC-ELECTRIC PROSPECTING Original FiledDec. 14, 1952 3 SheetsSheet l Fllhllllll Sefit. 15, 1936. I w BLAU AL2,054,067

METHOD AND APPARATUS FOR SEISMIC-ELECTRIC PROSPECTING Original FiledDec. 14, 1932 5 Sheets-Sheet 2 Sept. '15', 1936. L. w. BLAU ET AL METHODAND APPARATUS FOR SEISMIC-ELECTRIC P-ROSPECTING Original Filed Dec. 14,1932 3 Sheets-Sheet 3 transverse waves.

Patented Sept. 15, 1936 UNITED STATES PATENT OFFICE METHOD AND APPARATUSFOR SEISMIC- ELECTRIC PROSPECTING Original application December 14,1932, Serial No. 647,084. Divided and this application December 27,1934, Serial No. 759,348

28 Claims.

This invention relates to seismic-electric prospecting. Moreparticularly it relates to the use in seismic prospecting of the effectof seismic waves upon the electric constants of the ground.

This application is a division of our co-pending application Serial No.647,084 entitled Method and apparatus for seismic prospecting, filedDecember 14, 1932.

In seismic prospecting as previously practiced, vibrations are sent outfrom some source and recorded after refraction, reflection, etc., atboundary surfaces by means of seismographs located at any desireddistance from the source. In prospecting for oil, explosions of dynamiteare commonly used to generate the vibrations. It is well known thatearthquake shocks, both artificial and natural, are composed of severaltypes of vibrations or waves. The first waves to arrive at theseismograph are longitudinal waves in which the particles vibrate in thedirection of propagation of the wave. The transverse waves arrive next.In the transverse waves the particles vibrate in a plane perpendicularto the direction of propagation. The so-called Rayleigh and Love wavesare still slower than the In seismology, and especially in reflectionshooting, only times of arrival of the longitudinal waves are used bythe observer, but the reflections arriving after the direct longitudinalwaves are in general so distorted by the transverse waves, if these arenot eliminated, so as to make their identification impossible. It is inmany cases possible to eliminate or at least reduce the intensity of thetransverse waves since they are low frequency waves. In many areas,however, the higher frequencies of the longitudinal waves are absorbedleaving only their low frequency components. Any device or arrangementpreviously used which will eliminate or reduce the low frequencytransverse waves will also eliminate or reduce the low frequency.components of the longitudinal waves. Hence, if in such cases the highfrequencies are absorbed nothing or very little is left of thereflections. Therefore transverse waves cannot be eliminated bymechanical, acoustical or electriseismograph customarily used has adiameter of about three inches. Such a seismograph is responsive to waveenergy arriving over the area of the seismograph, say ten square inchesor less.

It is an object of this invention to obtain an integrated effect due toseismic waves propagated through a large volume of the crustof theearth. Another object is to provide an improved method and apparatus forcancelling out different waves; that is waves of different types andwave lengths for sufficient lengths of time to permit of the clearrecording of reflections.

Other objects will be apparent from the specification and from theaccompanying drawings in which latter- Fig. 1 is a diagrammatic view ofa preferred form of apparatus for carrying out the invention;

Fig. 2 is a top plan view of the apparatus i1- lustrated in Fig. 1; 20

Fig. 3 is a vertical sectional view through the ground showing apreferred arrangement of a plurality of receiving stations with respectto the shotpoint;

Fig. 4 shows a typical record obtainable through the use of theinvention;

Fig. 5 is a diagrammatic view of a modified form of the invention;

Fig. 6 is a top plan view ofthe apparatus illustrated in Fig. 5; and 30Fig. 7 is a diagrammatic view of still another form of the invention.

Referring particularly to Figs. 1 to 4 inclusive of the drawings,reference numeral I designates the shotpoint or source of disturbance of35 seismic waves. Reference numerals 3 and 4 are electrodes which extendinto the surface of the ground 6 in spaced relation to each other and tothe source I of seismic disturbance. The electrodes 3 and 4 areconnected in parallel by means of an electrically conductive line 8.Similarly electrodes 9 and disposed in the surface of the ground 6 inspaced relation to each other and to the source I of seismic energy areconnected in parallel by means of an electrically conductive line I2.Electrodes 3, 4, 9 and I0 may be non-polarizing or otherwise. A batteryI4 is connected in series with the primary I5 of a transformer I6 andwith the electrodes. The secondary I! of the transformer I6 is connectedto an appropriate amplifier Hi. The outlet leads of the amplifier l8 areconnected to an oscillo- -graph l9. Any even number of electrodes can beused if desired, such as two or more. The battery l8 causes a flow ofdirect current through the ground between the electrodes 3,, 4, 8 and 10thereby producing lines of electric force which approximately parallelthe lines 20. Some of these lines of force pass through the surfacelayer A directly between the electrodes. Other lines of force passthrough the subsurface strata B and C.

Preferably. a plurality of receiving stations designated X, Y and Z inFig. 3 are provided at different distances from the shotpoint and theeffects of the seismic disturbance at the different stations arerecorded simultaneously.

In the operation of the device, a seismic disturbance caused by anexplosion, falling weight or the like changes the electrical resistanceof the ground between the electrodes. The change in resistance causes avariation in the electrical current passing through primary l5 of thetransformer. This variation is impressed upon the amplifier l8 by thesecondary ll of the transformer and is finally recorded by theoscillograph 89. If desired, the battery or other source of electricalenergy It can be omitted. The natural.

something which has happened to the electric cur- 7 rent lines, eithernaturally or artificially, on the passage of seismic waves through thevolume or cubic contents of the earths strata through which theseelectric currents are flowing. Thus the direct wave and the surfacewaves which travel substantially along the surface of the earth 6 modifythese current lines for a relatively long time and therefore give a veryweak indication at the oscillograph or no indication at all, as comparedwith reflected waves 2i, see Fig. 3, arriving substantially in avertical direction from below which therefore modify or affect theelectric current lines almost simultaneously over the whole volume. Byway of specific example, the electrodes 3 and 4 may be disposed 100 feetapart from the electrodes 9 and I0. In such a case the current intensityat a depth of about 50 feet is quite appreciable. The electric currentlines then assume the form of a hemisphere with a radius of fifty feet.This hemisphere then has a volume of approximately 250,000 cubic feetand the seismic waves affect the electric current lines throughout andthe time interval is used between the explosion and a characteristicdeflection on the record.

To correlate these time intervals with deep reflecting horizons, a timedepth. curve is determined by setting up the receiving station near 0.

well, not shown, at the surface of the ground and by exploding dynamitecharges at different depths in" the W611. The time intervals arerelatively short.

By the procedure described an indication is obtained of a verticallytraveling wave 2| before the wave arrives at the surface 6 of the earthbecause the current lines 20 penetratelbelow the surface. In thepreferred procedure, the electrodes are separated to such a distancethat the current linespenetrate below the low velocity layer A. This isquite easy because the layers below the low velocity layer have arelatively high conductivity and also carry a relatively large naturalground current as compared with the low velocity layer. Thus many of theeffects of the low "velocity layer A' are eliminated.

Referring to Fig. 4, a record is shown which was obtained in prospectingan area in which the low velocity layer was only a few feet thick. Theelectrodes were disposed 25 feet apart. The record shows the integratedeffect of the seismic waves arriving in the large volume through whichthe current passed. The record shows the arrival of the reflected wavesfree from the low frequency waves and without the use of an electricalwave filter to eliminate the undesirable low frequency waves. The recordgives an indication of the wave before the wave arrives at the surfaceof the ground. The integrated efi'ect begins while the wave is still atsome depth below the surface of the ground.

While the invention has been described as recording changes in theelectrical resistance of the ground, it will be understood thatvariations due to a seismic disturbance in the capacity or inductancemay be utilized and such variations are included in the scope of theapplicants invention.

Referring particularly to Figs. 5 and 6, another means for utilizing theenergy arriving simultaneously over a very great vblume of the earth isillustrated. Reference numerals 23 and 28 designate electrodes which aredriven into the ground and are connected by means of electricallyconductive lines 25 to a battery 26. The distances between theelectrodes 23 and 24 may be from a few feet to hundreds of feet.

Additional electrodes 30 and 3| are driven into the ground at anyconvenient points on a straight line connecting the electrodes 23 and24. The electrodes 30 and 3| are connected through electricallyconductive lines 32 and 33 respectively to an amplifier 35 and anoscillograph 36 through a condenser 31 in the line 32. The electrodes23, 241, 30 and 3| may or may not be equidistantly spaced from eachother.

In the operation of the device, the ground in the vicinity of theelectrodes is caused to vibrate. by an explosion or other shock. As aresult, the

electrical resistance of the groundis changed.

This causes a change in the voltage drop across the electrodes 30 and3|, which change in voltage is impressed upon the amplifier 35, ismagnified by the amplifier and is finally recorded by the oscillograph36. The reflected waves arrive sub- 55 stantially in a verticaldirection from below through the volume through which the electriccurrents are flowing and therefore modify or affeet the electric currentover the entire volume,

thereby causing a large change in the voltage drop across electrodes 30and 3|. This is in contradistinction to the direct waves which progressvirtually along the surface of the ground and cause little change in thevoltage across the electrodes 30 and 3i.

Referring particularly to Fig. '7, a modified form of the invention isillustrated in which electrically conductive wires 4|] and 4| ofconvenient lengths are disposed upon the surface of or are buriedbeneath the surface of the ground extending approximately parallel toeach other. The wires are electrically insulated from the ground. Thewires 40 and 4| are spaced from each other suitable distances such as,for example, one foot. The wires 40 and 4| may be of any convenientlength such as 25 feet or 100 feet long. The wires 4|] and 4| areconnected across the grid 43 of a vacuum tube 44 in parallel with a coil45 and a variable condenser 46. This forms an oscillatory circuit, thefrequency of which varies with variations of the electrical constants.

The complete circuit includes a plate 48 and inductance 49 and avariable condenser 50. A

The wires 40 and 4| extend substantially radially from the shotpoint Iand are disposed parallel to a line connecting the instrument set-up wth the shotpoint. A wave traveling along the surface of the ground in adirection parallel to the wires 40 and 4| gives no indication upon theoscillograph 58. A wave traveling in a direction perpendicular to thewires 4|] and 4| does givean indication because such awave arrives atthe wires practically simultaneously along the whole length of thewires. Thus a direct wave gives no indication upon the oscillographwhereas a reflected wave does give an indication.

One of the wires can be insulated and the other wire bare of insulationwith the bare wire grounded and some of the advantages of theinventionwill be retained.

By the constructions described arrangements are used which respond towaves arriving over large volumes of the ground simultaneously. Thewaves arriving over areas several hundred feet long, such as 200 feet orlonger, are recorded. Thus the devices are adapted for the recording ofreflections. The electrical current lines are substantially horizontal.Consequently the receiving station is more sensitive to waves arrivingina vertical direction than to direct, and surface broadly as the priorart permits.

We claim:

1. The method of geophysical exploration which comprises receiving overa continuous extended volume of the ground including subsurface stratawave energy arriving throughout the volume from a. source of propagationof seismic waves, and obtaining an indication due to the eifect of thiswave energy on the electrical properties of the volume of ground.

2. The method of geophysical exploration, which comprises propagatingseismic waves from a given source, receiving over a continuous volume ofthe ground including subsurface strata extending radially from thesource wave energy arriving throughout the volume from the source, andobtaining an indication due to the effect of this wave energy on theelectrical properties of the volume of ground.

3. The method of geophysical exploration, 10 which comprises obtainingover a continuous extended volume of the earth an indication of theelectric constants of the earth, receiving over the volume wave energyarriving throughout the volume from a source of propagation of seismicwaves whereby said electric constants of the volume of earth arechanged, and obtaining an indication of the change of the electricalconstants.

4. The method of geophysical exploration which comprises passing anelectric current 20 through the ground between spaced electrodes,creating a seismic disturbance to change the electrical resistance ofthe ground, and recording the change in resistance.

5. The method of geophysical exploration, 25 which comprises passing anelectrical current through the ground between a plurality of spacedpositive electrodes connected in parallel and a plurality of spacednegative electrodes connected in parallel, creating a seismicdisturbance to change the electrical resistance of'the ground, andrecording the change in resistance.

6. The method of geophysical exploration, which comprises conducting thenatural ground current through spaced positive electrodes connected inparallel and spaced negative electrodes connected in parallel, creatinga seismic disturbance to change the natural ground current, andrecording the change of current.

'7. The method of geophysical exploration, which comprises passing anelectric current through the ground from spaced electrodes, conductingthe resulting ground current through additional spaced electrodessubstantially aligned with the first mentioned electrodes, creating aseismic disturbance to change the electrical resistance of the groundwhereby a change in voltage drop is caused across the additionalelectrodes, and recording the change in voltage.

8. The method of geophysical exploration, which comprises propagatingseismic waves from a given source, disposing-two parallel wires alongthe'ground substantially radially from the source and at a point removedfrom the center of disturbance, and recording the changes in electricalproperties of the ground due to the arrival of the seismic waves.

9. The method of geophysical exploration, which comprises propagatingseismic waves from a given source, disposing a plurality of parallelwires along the ground substantially radially from the source and at apoint removed from the center of the' disturbance, and recording thechanges in electrical properties of the ground between and aroundalternate wires, due to the arrival of the seismic waves.

10. The method of geophysical exploration, which comprises propagatingseismic waves from a given source, disposing a. plurality of insulatedwires, each connected together along the ground, distributed inanyconfiguration at a point removed from the center of disturbance, andrecording the changes in inductance, capacity, resistance, and otherelectrical properties of the ground around the wires due to the arrivalof the seismic waves.

11. In a system for geophysical exploration, means adapted to bedisposed in an extended volume of the earth operative to receive waveenergy arriving throughout the volume from a source of propagation ofseismic waves, and indicating means operatively associated with thereceiving means'responsive to the seismic wave energy which has arrivedover this extended volume.

12. The method of geophysical exploration, which comprises receivingmore plurality of extendedvolumes of the ground including subsurfacestrata wave energy arriving throughout the volumes from a source ofpropagation of seismic waves, the volumes being disposed at differentdistances from the source, and simultaneously obtaining indications dueto the eilect of this wave energy on the electrical properties of thevolumes of the ground. I 13. In a system for geophysical exploration, asource of propagation of seismic waves, means adaptedto be disposed inan extended elongated volume of the earth radially of the source andoperative to receive wave energy arriving throughout the volume from thesource, and an indicating means operatively associated with thereceiving means responsive to the seismic wave energy which has arrivedover this extended volume.

14. In a system for geophysical exploration, electrical means adapted tobe disposed in an extended volume of the earth responsive to changes inthe electric constants of the earth over said volume, operative toreceive wave energy arriving throughout the volume from a source ofpropagation of seismic waves which modify the electric constant of theearth, and an indicating means operatively associated with the receivingmeans responsive to the seismic wave energy which has arrived over thisextended volume.

15. In a system for recording seismic disturbances, spaced electrodes inthe ground spaced from a. source of seismic waves, means for passing anelectric current through the ground from the spaced electrodes, andmeans for recording variations in the current caused by the seismicwaves.

16. In a system for recording seismic disturbances, a group of spacedelectrodes in the ground connected in parallel, a second group of spacedelectrodes in the ground connected in parallel, a transformer theprimary of which is connected to the electrodes, an amplifier connectedto the secondary of the transformer, and an oscillograph connected tothe output leads of the amplifier.

17. In a system for recording seismic disturbances, spaced electrodes inthe ground, means for passing an electric current through the groundbetween the electrodes, additional spaced electrodes in the groundsubstantially aligned with the first mentioned electrodes so that aseismic disturbance changes the electrical resistance of the groundwhereby a change in voltage drop is caused across the additionalelectrodes, and an electric circuit connected to the additionalelectrodes including means for recording the drop.

18. In a system for recording seismic disturbances, spaced electrodes inthe ground, means for passing an electric current through the groundbetween the electrodes, additional spaced electrodes in the groundbetween and substantially aligned with the first mentioned electrodes,so that a seismic disturbance changes the electrical resistance of theground whereby a change in voltage drop is caused across the additionalelectrodes, and an electric circuit including a recorder connected tothe additional electrodes for recording the drop in voltage.

19. In a system for. recording seismic disturbances, a, vacuum tube,parallel wires on the ground spaced and extending radially from thesource and connected across the grid and filament of the oscillatorvacuum tube, the plate current of which is changed by variations of theelectrical properties oi. the ground between and around the wires causedby the arrival of seismic waves, suitable amplification, a transformerand oscillograph for recording the vibrations so pro- 20. In a systemfor geophysical exploration, 8. source of seismic waves, parallel wiresburied beneath the surface of the ground spaced and extending radiallyfrom the source, means for recording the changes in electricalproperties of the ground between and around the wires caused by thearrival of the seismic waves.

21. The method of geophysical exploration which comprises receivingthrough a volume of the ground the wave energy arriving through thevolume from a source of propagation of seismic waves, and obtaining anindication due to the effect of this wave energy on the electricalproperties of the volume.

22. The method of geophysical exploration which comprises passing anelectric current through the ground between spaced electrodes, creatinga seismic disturbance to change the electrical impedance of the ground,and recording the change in impedance.

23. Apparatus for geophysical exploration,

which comprises means for passing an electric current througha givenvoluineof the ground, means for creating seismic disturbances to changethe electrical impedance of the volume of the ground, and means forrecording the change in impedance.

24. The method of geophysical exploration, which comprises conductingthe natural ground currents through the ground between spacedelectrodes, creating a seismic disturbance to change the natural groundcurrent, and recording the change of current.

25. Apparatus for geophysical exploration, which comprises spacedelectrodes in the ground, means for conducting the natural groundcurrent through the ground between the electrodes, means for creating aseismic disturbance to change the natural ground current, and means forrecording the change of current.

26. In a system for recording seismic disturbances, spacedelectrodes inthe ground, means for passing an electric current through the groundbetween the electrodes, additional spaced electrodes in the groundsubstantially aligned with the first mentioned electrodes so that aseismic disturbance changes the electrical impedance of the groundwhereby a change in voltage drop is caused across the additionalelectrodes, and an electric circuit connected to the additionalelectrodes including means for recording the drop.

27. In an apparatus for measuring changes in electrical impedance of theground due to seismic disturbances, means adapted to be disposed over anextended volume of the ground responsive to changes in the electricalimpedance of the ground over said volume operative to receive the waveenergy arriving throughout the volumefrom a source of propagation ofseismic waves which modify the electric impedance of the ground, and anelectric circuit connected to the means and including a transformer, anamplifier and a recorder whereby changes in the electric impedance tweenthe electrodes through an extended volume of the ground are recorded. ofthe ground and the amount of electric energy 28. In an apparatus formeasuring changes in flowing; through the volume is changed by theelectrical impedance of the ground due to seismic seismic disturbances,means connected to the disturbances, spaced electrodes in the ground, ansecondary of the transformer for amplifying the 5 electric circuitconnecting the electrodes including change in current, and means forrecording the a source of electric energy and the primary 0! a amplifiedchange.

transformer connected in series with the elec- LUDWIG W. BLAU.

trodes whereby lines of electric force pass be- LOUIS STATHAM.

